July 15, 1915 Methods of Bacterial Analyses of Air 357 



COMPARISON OF RETTGER AEROSCOPE WITH THREE TYPES OF THE MODIFIED STANDARD 



AEROSCOPE 



In order to ascertain, if possible, the reason why the Rettger aero- 

 scopes had given such decidedly lower results in the previous series 

 of analyses than the sand filters, another series of analyses was made. 

 From the data already secured it did not seem probable that the lower 

 figures could be explained entirely by loss due to the clinging of bacteria 

 to the entrance tube. For that reason a series of tests was planned in 

 order to discover whether the difference in the size of the entrance tubes 

 had any effect on the number of bacteria caught in the filters. Four 

 types of aeroscopes were used: (i) The Rettger aeroscope; (2) the 

 modified standard aeroscope used in the preceding experiments; (3) an 

 aeroscope of the same type as No. 2 except that the entrance tube was 

 of the same diameter and length as the one used in the Rettger aero- 

 scope; and (4) an aeroscope of the same type as the second except that 

 the top was left wide open when the air was being drawn through it, 

 precipitation of bacteria and dust being prevented by means of a screen 

 placed above it. 



Ten liters of air were drawn through each aeroscope at the rate of X 

 liter per minute. Five c. c. of sterile water was used in the Rettger 

 aeroscope as the filtering agent and the same quantity of sterile water 

 was used in making the bacterial suspensions of the sand in the other 

 aeroscopes. Plating was done in triplicate in the usual manner, the 

 entrance tubes to the Rettger aeroscopes being more thoroughly rinsed 

 (about 1 2 times) than had previously been done. The results of 2 1 such 

 comparative tests computed on the basis of per liter counts are given 

 in Table V. The averages of the tests showed that the plates made 

 from the Rettger aeroscopes (column i ) developed 90 colonies per liter, 

 those from the modified standard aeroscopes (column 2) developed 90 

 per liter, those from the modified aeroscopes with the small opening 

 (column 3) developed 97, while those from the modified aeroscope pro- 

 tected by a shield (column 4) developed 80 per liter. Individual results 

 varied greatly, sometimes one aeroscope and sometimes another giving 

 the highest numbers. From this series of analyses it seems evident that 

 the size of the opening has little influence upon the number of bacteria 

 caught by the filters, and it also shows that under some conditions the 

 Rettger aeroscope may catch as many bacteria as the sand filters. Just 

 why the Rettger aeroscope should have proved more efficient in the 

 series of tests given in Table V than in the series given in Table IV is 

 not evident from the analyses given, unless it was due to the more 

 thorough rinsing of the inlet tube. 



